Attempts have been made for many years to improve, and especially to automatize, the setting and control of the essential operating parameters, especially color setting, of printing machines. Typical examples of the corresponding efforts emerge from patents GB-OS 2,165,801, DE-OS 33,14,333, DE-OS 38,06,100, DE-OS 37,35,785, DE-OS 39,29,085, DD-PS 236,292, DD-PS 256,291, GB-OS 2,064,113, DE-PS 32,04,501, DE-OS 37,30,625 and EP-PS 61,596.
In these cases, essentially measurement and control elements which are dependent on the printed original are used. In order to make possible an exact as possible effect of the color control per color point zone of the printing machine, these measurement and control elements are combined into one setting for each color point zone and into so-called control strips over the printing width. These strips are arranged outside of the printing image in the "white" edge of the printed sheet. These control strips are scanned either on-line, that is, directly in the running printing machine, or off-line on the material to be printed, i.e., printing samples are examined outside of the printing machine, so that corresponding adjustments can then be made.
However, the use of measurement and control elements that are dependent on the printed original have the disadvantage that, by this means, no immediate connection with the tolerance-sensitive image points of the printed originals can be achieved. Consequently, more control elements are used per printed page than would be actually necessary, and there are also no set values for these elements which are specific to the printed original. In addition, the management of measurement and control elements necessitates not inconsiderable expense for the printing block preparation.
For eliminating these shortcomings, various suggestions have now been made in order to determine, directly from the printing format, the factors which are decisive for quality, especially halftone dot size and full-tone density, without specific control strips.
EP-OS 0,136,642 deals with a process for determining the full-tone density and the point increment on two three-color, halftone elements by means of densitometric measurements, calculation formulas and calibration curves. This process is very expensive and is realized in practice only with extremely great expenditure.
Further, from DE-OS 39,25,011 emerges a process for checking the printing quality of multi-colored printed originals of an offset printing machine, in which, with the aid of a camera with color-selective sensor fields, color separation images of the image segment to be examined are produced in the spectral regions red, green and blue, and the color separation images are processed by means of methods of image analysis in an image processing system, such that the color layer thickness of the color halftone dots of the individual primary colors can be determined via a halftone measurement. This process takes into account the desired average color halftone dot density in the measurement segment, but does not consider the tolerances permissible based on the perceptibility of the human eye.
DE-OS 28,29,341 deals with the data entry for color control units, in which a manual densitometer is connected to a color control unit. For the simplification of operation, a user guidance is provided by a display device connected to the densitometer. With the manual densitometer, measurements can be made on a printed sheet in freely selectable image zones, which are adapted to the subject. Since the measurement points can be located in the image area, the above-mentioned color control strips are unnecessary. However, it is not described how these image zones and their appropriate set values are determined.
The process of pulling printed sheets for random samples with regard to process parameters which are decisive for quality, taking measurements, and with the aid of the results found, carrying out point operations according to the usual methods of control and regulation technique is known from DE-OS 2,728,738.
Further, from U.S. Pat. No. 4,468,692 emerges a process for varying the colors of an image, in which density signals for the colors cyan, magenta, yellow and black, which are taken from the original, are supplied to the color display. Color signals for cyan, magenta and yellow are converted by using tabular memories in color separation density signals for red, green and blue, the converted color separation density signals are then individually added.
Subsequently, correction values for the printing of the colors, which are obtained from a second tabular memory depending on the color signals, are added superimposed to the added color separation density signals. A correction value for the printing of black onto the other colors, which is read out from a third tabular memory, can be added to the signals received in this manner.
Further, DE-OS 33,19,941 demonstrates a process for the display of a color image simulating a multi-color printing on a display screen in which data, which is determined for the production of a multi-color printing, is polled from a memory device and is selectively added for all printing colors, that is, for the production of a density signal, which, after its processing, controls a color kinescope for the display of the color image simulating the multi-color printing.
U.S. Pat. No. 4,414,635 demonstrates a color control simulator, in which a variation of the various color signals can be made by means of tabular memories.
From U.S. Pat. No. 4,843,379 is known a monitor for the reproduction of color images, in which the saturation of image colors is affected by means of a predetermined algorithm, while the color tone and the color intensity remain unaffected.
Japanese Patent Application 1-131,568, published in Patent Abstracts of Japan, P-922, on Aug. 22, 1989, vol. 13/No. 377, demonstrates a printing simulator, in which signals are digitally processed and stored for the colors yellow, magenta and black. The output signals of the memory are switched such that the signals read out are displayed half in horizontal lines and half in vertical lines. Therefore, for example, two images can be displayed by two memories at the same time and can be compared to one another on the display screen of the color monitor.
Further, an attempt has already been made to analyze and arrange the permissible tolerance as a function of the printed original. In this regard, DE-OS 35,43,444 describes a process, in which halftone dot sizes, possibly even full-tone densities, are determined on measurement fields printed together within the color zones, and with the dropping out of same, the printing process is engaged from tolerance ranges assigned to them, correcting by actuation of final control elements. In this case, each printing job is classified by means of a catalog of typical test images and color tables into an image contrast class. Based on the classification into this class, the tolerance to be observed can subsequently be set. Thus, the corresponding set values apply to the image-independent, standardized control elements for full and matrix densities. However, this process has the following disadvantages: For each printed original, a test image, must first be sought and found; if, with the aid of this test image, it turns out that the tolerance scope is too narrow for the printing process provided, it is difficult to estimate how the printed original reacts upon "outbidding" the tolerances specific to the printing process, that is, when the limits of the tolerance range are reached. In addition, this process does not take into account the different multi-colored superstructures of the color separations, and finally, it is very expensive if the printed originals must be classified with reference to different color zone sensitivities.
A further development of this process is described in DE-OS 36,04,222, according to which measurement fields are formed in selected color zones in the form of combination measurement fields, For this purpose, single color measurement fields of at least two different printing colors are printed superimposed in order to be able to reduce the number of measurement elements per color zone in the control strips. Corrected values for the full-tone densities and/or halftone dot sizes are obtained from the measured values determined in these combination measurement fields. The inclusion of the permissible tolerance range and the prior checking of the effects of fluctuations of the operation parameters in this tolerance range are not visually possible.